The present invention is directed to the design of an implantable defibrillator patch electrode for use with a cardiac pacing and defibrillating device. More particularly, the present invention is directed to the design of patch electrodes which are secured to the exterior surface of the heart to deliver an electrical stimulus to the heart to cause defibrillation.
Tachycardia is a condition in which the atria, ventricles or both chambers of the heart, beat very rapidly, and not within the normal physiological range, typically exceeding 160 occurrences per minutes. Atrial tachycardia is the medical term assigned to the condition in which rapid and regular succession of P-waves of the PQRST waveform complex occur. The rate of occurrence is in excess of the physiological range normally encountered in the particular patient.
Paroxysmal supra-ventricular tachycardia is the medical term assigned to the condition in which there is a sudden attack of rapid heart condition in the atria or in the atrial-ventricular node. The characteristics are generally the same as those in atrial tachycardia.
Normally atrial tachycardia and paroxysmal supra-ventricular tachycardia are not a life-threatening condition, unless they progress into ventricular tachycardia or fibrillation. Ventricular tachycardia is the medical term assigned to the condition in which rapid and regular succession of R-waves of the QRST waveform complex occur. Again, the rate of occurrence is in excess of the physiological range of the particular patient and, if left untreated, can progress into ventricular fibrillation. In ventricular fibrillation, the ventricles are unable to pump blood in a coordinated fashion and cardiac output drops to a level dangerous to the patient.
Typically, these life-threatening conditions of ventricular tachycardia or ventricular fibrillation must be treated by drug therapy or by electrical stimulation, such as cardioversion or defibrillation. Implantable defibrillators have been developed to monitor the pacing of the heart, and provide a defibrillation charge via an electrode attached to or implanted in the heart. These implantable defibrillators allow the recipient a considerable degree of freedom to pursue normal activities with the defibrillator monitoring the rate of the heart and providing a defibrillation charge immediately upon onset of ventricular tachycardia.
Accordingly, for certain patients, it is beneficial to affix to the exterior surface of the cardiac muscle a patch electrode which, when electrically connected to an electrical power source, can deliver a large electrical stimulus directly to the cardiac muscle to cause defibrillation. The electrical energy necessary for defibrillation when delivered by an implanted patch electrode may be in the range of, for example, between 1 and 100 joules, but is preferably in the range of between 5 and 30 joules. It is important to recognize that when this amount of energy is being coupled directly to the cardiac muscle, there is a potential for severe damage to the tissue. If such damage occurs, the efficiency of the defibrillation for the patch electrode in a subsequent application may be severely impaired.
In addition, the design of the patch electrode must allow intimate contact over a substantial surface of the cardiac muscle and effective delivery of the defibrillation energy. A further consideration of the design of the patch electrode requires, given its location on the surface of the continuously flexing cardiac muscle, that the patch electrode itself is extremely flexible and resistant to fatigue.
With the foregoing in mind, a patch electrode is normally designed as a metallic mesh positioned between a polymer insulating backing and an insulating frame. The patch shapes which have been used include both oval shapes and rectangular shapes. Generally, an oval shape allows more intimate contact with the surface of the heart muscle. The insulating backing is bonded to the metallic mesh and operates to direct the defibrillation current toward the opposite side of the patch electrode, into the cardiac muscle. One problem associated with bonding the wire mesh of the electrode to the insulating backing is the potential for delamination and associated tissue ingrowth. One design which reduces this problem on a rectangular patch is illustrated in Design Patent No. 273,514, wherein the rectangular patch electrode is overlaid with an insulating lattice which divides the rectangular patch electrode into six squares. The lattice may assist in preventing delamination between the insulating backing and the wire mesh of the patch electrode.
A common problem with metallic patch electrodes is the high current density around the perimeter which may be referred to as fringing or edge effect. This effect can cause an uneven current distribution to the cardiac muscle, resulting in high defibrillation threshold levels and tissue damage proximate the edge of the patch electrode. It would therefore be desirable to provide an even current or polarization distribution for a patch electrode.